title: Mitsugumin 23 forms a massive bowl-shaped assembly and cation-conducting channel.
abstract: Mitsugumin 23 (MG23) is a 23 kDa transmembrane protein localized to the sarcoplasmic/endoplasmic reticulum and nuclear membranes in a wide variety of cells. Although the characteristics imply the participation in a fundamental function in intracellular membrane systems, the physiological role of MG23 is unknown. Here we report the biochemical and biophysical characterization of MG23. Hydropathicity profile and limited proteolytic analysis proposed three transmembrane segments in the MG23 primary structure. Chemical cross-linking analysis suggested a homo-oligomeric assembly of MG23. Ultrastructural observations detected a large symmetrical particle as the predominant component and a small asymmetric assembly as the second major component in highly purified MG23 preparations. Single-particle three-dimensional reconstruction revealed that MG23 forms a large bowl-shaped complex equipped with a putative central pore, which is considered an assembly of the small asymmetric subunit. After reconstitution into planar phospholipid bilayers, purified MG23 behaved as a voltage-dependent, cation-conducting channel, permeable to both K(+) and Ca(2+). A feature of MG23 gating was that multiple channels always appeared to be gating together in the bilayer. Our observations suggest that the bowl-shaped MG23 can transiently assemble and disassemble. These building transitions may underlie the unusual channel gating behavior of MG23 and allow rapid cationic flux across intracellular membrane systems.
authors.affiliation: School of Physiology and Pharmacology, Bristol Heart Institute and Centre for Nanoscience and Quantum Information, University of Bristol, Bristol, United Kingdom.
authors.forename: Elisa
authors.initials: E
authors.lastname: Venturi
authors.forename: Kazuhiro
authors.initials: K
authors.lastname: Mio
authors.forename: Miyuki
authors.initials: M
authors.lastname: Nishi
authors.forename: Toshihiko
authors.initials: T
authors.lastname: Ogura
authors.forename: Toshio
authors.initials: T
authors.lastname: Moriya
authors.forename: Samantha J
authors.initials: SJ
authors.lastname: Pitt
authors.forename: Kazutaka
authors.initials: K
authors.lastname: Okuda
authors.forename: Sho
authors.initials: S
authors.lastname: Kakizawa
authors.forename: Rebecca
authors.initials: R
authors.lastname: Sitsapesan
authors.forename: Chikara
authors.initials: C
authors.lastname: Sato
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Takeshima
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism; D003432:Cross-Linking Reagents / Q000737:chemistry; D004721:Endoplasmic Reticulum / Q000201:enzymology; D006801:Humans; D057927:Hydrophobic and Hydrophilic Interactions; D021621:Imaging, Three-Dimensional; D007473:Ion Channels / Q000737:chemistry* / Q000302:isolation & purification / Q000378:metabolism* / Q000648:ultrastructure; D008051:Lipid Bilayers / Q000378:metabolism; D008565:Membrane Proteins / Q000737:chemistry* / Q000302:isolation & purification / Q000378:metabolism* / Q000648:ultrastructure; D008854:Microscopy, Electron; D010446:Peptide Fragments / Q000737:chemistry; D011188:Potassium / Q000378:metabolism; D011487:Protein Conformation; D021122:Protein Subunits / Q000737:chemistry / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism; D011817:Rabbits; D011994:Recombinant Proteins / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism / Q000648:ultrastructure; D012519:Sarcoplasmic Reticulum / Q000201:enzymology; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid
chemical_list: D003432:Cross-Linking Reagents; D007473:Ion Channels; D008051:Lipid Bilayers; D008565:Membrane Proteins; D010446:Peptide Fragments; D021122:Protein Subunits; D011994:Recombinant Proteins; C547817:mitsugumin23 protein, human; D011188:Potassium; D002118:Calcium